Power generating systems have been devised wherein variable speed motive power produced by a prime mover is converted into constant frequency electrical power which is supplied to one or more loads. Typically, the conversion is effected by a synchronous generator driven by a constant speed drive which is in turn coupled to the output of the prime mover. An alternative to such a generating system is known as a variable speed constant frequency (VSCF) generating system that includes a synchronous generator which receives the variable speed motive power produced by the prime mover and a power converter which converts the resulting variable frequency output of the generator into constant frequency power for the loads. Interest in VSCF systems has increased of late owing to the push to design "all electric" aircraft in which the use of mechanical, hydromechanical and hydraulic components is minimized.
It has been recognized that the generator in a generating system can be used in a starting mode to provide motive power to the prime mover to accelerate the latter up to self-sustaining speed, at which point a self-sustaining condition may be achieved. Such a system is disclosed in a paper entitled "DC Link VSCF Starter/Generator Systems", by D. E.. Baker, SAE Technical Paper Series No. 871887, copyright 1987. In the system described in this paper, external power is provided to the power converter which in turn is controlled to provide electrical power to the generator. The generator is thus operated as a motor to provide the required motive starting power to the prime mover.
A further power generating system which is operable in a starting mode to provide motive power to a prime mover is disclosed in Mehl et al. U.S. Pat. No. 4,481,459.
During operating in the starting mode, one or more malfunctions may occur which can cause damage to or destruction of system components. For example, the prime mover may fail to ignite (or "light-off") and thus fail to reach a self-sustaining condition. As a result of such malfunction, the generator may be operated to provide motive power to the prime mover for an excessive period of time, in turn leading to potential damage.
Alternatively, the generator may become disconnected from the prime mover due to breakage of a shaft, failure of a gear box between the prime mover and generator or for another reason. In this case, the generator will experience a low inertia condition during the start mode and may accelerate rapidly to potentially damaging speeds.
In addition to the foregoing, the prime mover may stall during operation in the start mode. This can result in the generator failing to accelerate the prime mover to the self-sustaining speed within a preselected maximum starting period. Such a condition can also result in damage to the generator as well as to electrical system components in the converter.
For the foregoing reasons, it is desirable to detect a malfunction when operating in the starting mode at the earliest possible time so that the starting mode operation can be terminated to prevent excessive wear and damage to the generator or other components.
Kuntner et al. U.S. Pat. No. 4,288,828 discloses a protection system for an electric motor which drives high inertia loads that require relatively long acceleration times. The protection system deenergizes the motor when the motor exceeds a preset time limit to reach normal running speed. The protection system includes a timer which is enabled upon a start command from an operator. During subsequent acceleration of the motor, current sensing transformers connected to the motor develop a speed signal representing the speed of the motor. A comparator compares the speed signal against a reference signal which represents the normal running speed and develops an up-to-speed signal at an output when the speed signal reaches the reference signal. If the motor reaches normal running speed within the preset time limit, the up-to-speed signal resets the timer before expiration thereof to prevent the timer from developing an output representing an error condition.
The above-identified Kuntner et al. patent does not detect the failure of a prime mover to attain a self-sustaining speed during a start mode, a low inertia condition caused by a broken shaft or gear box or a prime mover stall condition.